help understanding this hbridge design....

hello all once again! today I am having some problems with understanding how to drive this particular h bridge design, because I don't fully understand how this H-bridge works, what its limitations are, etc.

The attached PDF is of close resemblance to my current config, sorry about the schematic being so bunched together, I drew it up quick with little regard to others ever viewing...the attached GIF is a very close variation (different poles and parts). The particular part choice used was taken from the cheapo radioshack rc truck pcb that I copied the H bridge design from originally. I have it replicated on a breadboard right now with changes made to the small signal transistor- in their place I have substituted with photocouplers (PC817) to isolate the pic16f876A (16f886 in final design due to its 4pwm's and many adc's) from the H bridge(s) [the second h is for the steering motor- its a cheapo radioshack servo delux!(dc motor, mechanical slide switch)] noisy switching, etc. So in essence the photocouplers on the 'big H' are driving the larger transistors, or as far as I interpreted it?

What confuses me with the big H, as compared with a more typical design like the second small signal transistor h bridge in the pdf, is the use of those additional transistors to apparently drive the larger pairs--in the original radioshack implementation, I do not believe that it was pwm'd signals from the controlling IC (that is just my guess, couldn't verify cause I scooped some parts from the pcb for my breadboard!) but rather used to simply switch stready high or low to change directions--no speed control from radio shack toys...SO, will this copied design present a problem if I was intending to use pwm from the PIC to control speed of the rear drive motor? OR, will it be no problem since the transistors they used are also able to switch up to certain freq- I just would need to figure out how to pwm from pic at suitable specs from the pwm modules?

Is this design able to produce both speed control and direction control using only the twp pins from the pic, or would it need to be modified (ie. pwm speed control, gpio for direction switching poles?)

Also, I wonder is it too much to ask for a simple code snippet (targetted for pic16f876 or 886 preferably but...) that would be able to drive this H config using two pic signals only if it is indeed a good design and able to be driven without additional circuits. As for the schematic, please ignore the whole feedback system, it is not in play at the moment and still in total rough anyways...

Thanks in advance for any tips, especially for some code example, I just want to make sure the bridge works to control both dir and speed before I finish the rest of the tweaks and go onto the feedback design...regards, jd

Please do not make duplicate posts or threads, they do not increase your chance of a helpful response but only reduce it, as people get confused and can't always see what others have suggested in the other thread(s).

If you have made this post or thread in error then please do not be offended. We get lots of people making duplicate threads under the misapprehension that it will increase their chance of a helpful response when in practice the reverse is true.

oops, wrong .gif!!

Ok, I realized that I posted the wrong schematic- please find attached proper schematic which should bring better sense to my previous post!

Well still trying to figure out the big H- I managed to find some great info regarding a very similar implementation which gives detailed operating description of the similar design- their implementation differs though because each pow trans is switched by a small signal transistor (well actually optoisolators but same diff ). The control config is also different in that there is two dir signals and an enable.

Back to the posted design, I am going through microchip appnotes right now which are also very informative- from them I now gather that I can indeed drive the posted design with both direction and speed control without modifying the schematic further- I had been not thinking correctly about what pwm does and what it is intended to accomplish. But basically I think I am able to use pwm to accomplish the tasks at hand, so off I go.